Method of making non sag incandescent tungsten filament

ABSTRACT

Incandescent lamp incorporates a non-sag tungsten filament having a microstructure comprising a plurality of elongated and interlocking grains disposed along the axial dimension of the filament. A large number of very minute inert gas bubbles, such as helium bubbles, are included within the filament, and a portion of the bubbles are generally aligned along the axial dimension of the filament and are positioned at the boundaries of the interlocking grains which comprise the filament. In the method for making the filament, inert gas atoms, such as helium atoms, are formed within the tungsten and when the filament is incandesced, the atoms migrate to form the very minute helium bubbles.

llnited States Patent [19] Sell et a1.

[11] 3,820,868 June 28, 1974- METHOD OF MAKING NON-SAG INCANDESCENT TUNGSTEN FILAMENT [75] Inventors: Heinz G. Sell, Cedar Grove, N..l.;

Heinz-J. Stepper, Konigsbrunn, Germany v [73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa. [22] Filed: Nov. 24, 1972 [21] Appl. No: 309,590

Related US. Application Data [62] Division of Ser. No. 217,162, Jan. 12, 1972.

[56] I References Cited UNITED STATES PATENTS Cheney eta1..... 148/126 Primary Examiner-Roy Lake Assistant Examiner-J. W. Davie Attorney, Agent, or Firm-W. D. Palmer [57] ABSTRACT Incandescent lamp incorporates a non-sag tungsten filament having a microstructure comprising a plurality of elongated and interlocking grains disposed along the axial dimension of the filament. A large number of very minute inert gas bubbles, such as helium bubbles, are included within the filament, and a'portion of the bubbles are generally aligned along the axial dimension of the filament and are positioned at the boundaries of the interlocking grains which comprise the filament. In the method for makingthe filament, inert gas atoms, such as helium atoms, are formed within the tungsten and when the filament is incandesced, the atoms migrate to form the very minute'helium bubbles.

4 Claims, 4 Drawing Figures PATENIEDJIIIZB i874 SHEU 1 BF 2 FIG.2.

METHOD OF MAKING NON-SAG INCANDESCENT TUNGSTEN FILAMENT This is a division of application Ser. No. 217,162 filed Jan. 12, 1972. I

BACKGROUND OF THE INVENTION This invention generally relates to incandescent lamps and, more particularly, to an incandescent lamp which incorporates an improved non-sag tungsten filament/There is also provided a method for making the filament. I

Non-sag tungsten filaments were initially developed prior to 1920 and the introduction of the non-sag tungsten filament, as described in US. Pat. No. 1,410,499 dated Mar. 21, 1922, had a great impact on the incandescent lamp industry. In explanation of the term nonsag, if a tungsten filament elongates when it is operated, and thus sags, the individual turns between filament coils will tend to contact one another and short out, thereby shortening the life of the filament. In the practices of the prior art, non-sag tungsten has been fabricated by including therein a very small amount of so-called dopant, which dopant is added in the form of alkali silicates. Because of this dopant addition, when the filament is initially incandesced, the crystals or grains which are formed therein are elongated and interlocking and generally disposed along the axial dimension of the filament. Within recent years, it has been discovered that the non-sag characteristic which is imparted by the alkali silicate doping is due to the formation of very minute potassium particles, which form potassium bubbles when the filament is incandesced. A substantial portion of these bubbles are generally aligned along the axial dimension of the filament and serve to inhibit the recrystallization and control the grain growth of the filament due to dislocation and grain boundary pinning by the potassiumv bubbles.

These potassium-formed bubbles are generally submicroscopic in nature and a representative diameter of same is in the order of 100 to 500 A. While the performance of such a filament is generally satisfactory, a major problem in the manufacture of the tungsten is the uniform incorporation of the doping impurity into the metal, and the attainment of the consistency of the required concentration, in order to produce the best filament material.

It is disclosed by C. E. Ellis in ACTA Metallurgica, Volume 11, February 1963 that helium filled bubbles formed in samples of cold-worked aluminum by cyclotron alpha irradiation have an effect'on subsequent re crystallation and grain growth in the aluminum samples. The helium bubbles also inhibit grain growth in beryllium and recrystallization in zirconium.

SUMMARY OF THE INVENTION In accordance with the present invention, there is provided, in combination with an incandescent lamp, an elongated tungsten filament having a microstructure which comprises a plurality. of elongated and interlocking grains defined by boundries therebetween. The elongated dimension of the grains is disposed along the axial dimension of the filament. A very large number of very minute, individual inert gas bubbles are included within the filament, and a portion of the individual bublocking grains. The resulting structure operates with a non-sag characteristic.

In processing tungsten, the material is initially formed into a relatively massive ingot by conventional selfresistance sintering techniques. Thereafter, the relatively massive ingot is mechanically reduced in cross section, first by swaging and then by drawing into filamentary form. In accordance with the method for making the present filament, after the relatively massive sintered ingot has been formed, and before the resulting elongated tungsten filamentary member has been heated to a condition of incandescence, there is formed within the tungsten body a very large number of inert gas atoms. Thereafter, after the elongated filamentary member has been fabricated into the form intended for its ultimate use, such as a coil or a coiled-coil, and also mounted in the environment in which it is intended to operate, the filamentary member is heated to a condition of incandescence which causes the inert gas atoms to coalesce into a very large number of very minute inert gas bubbles, which causes the filament to recrystallize with an interlocking and non-sag grain structure. In one method for forming the inert gas atoms within the tungsten body, a small amount of B is included in the tungsten and it is bombarded with neutrons after reduction to a relatively small diameter, in order to form helium atoms and alkali metal atoms. In another method for forming the helium atoms within the tungsten, the tungsten filament is irradiated with alpha particles in order to form helium atoms. Thereaf ter, when the filament is incandesced, the helium atoms coalesce to form helium bubbles.

I BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the invention, reference should be had to the preferred embodiment, ex-

bles are generally aligned along the axial dimension of the filament and positioned at boundries of the interemplary of the invention, shown in the accompanying drawings in which:

FIG. 1 is an elevational view, partly in section, illustrating an incandescent lamp which incorporates the non-sag tungsten filament which has been processed in accordance with the present invention;

FIG. 2 is a greatly enlarged sketch showing a crosssection of a recrystallized tungsten filament which includes helium bubbles in the body thereof, with a portion of the bubbles positioned at the boundries of the overlapping grains which comprise the filament;

FIG. 3 is a transmission electron micrograph of alpha-particle irradiated pure tungsten annealed at 1,900C for 45 minutes showing helium bubbles having a diameter of approximately 200A; and

FIG. 4 is a photomicrograph (taken at 1,000 X) of alpha-particle irradiated pure tungsten ribbon annealed at 2,200C for 45 minutes showing the elongated, interlocking grain structure developed in the irradiated layer.

DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in FIG. 1, the generally conventional incandescent lamp 10 comprises a sealed radiationtransmitting'envelope 12 which encloses a predetermined environment, such as an argon atmosphere, in which an incandescent filament 14 can be operated. Lead-in conductors 16 are sealed through and extend into the envelope 12 and these electrically connect to a conventional standard screw base 18. The envelope 12 can carry thereon an internal coating 20 of lightscattering material, if desired. The filament 14 has been fabricated in accordance with the teachings of the present invention, as explained in detail hereinafter. The filament 14 can be formed as a single coil of tungsten wire or as a coil which in turn is formed into a coil to form a so-called coiled-coil. Of course, other filament constructions are possible, as are well known in the art and the gaseous atmosphere can be replaced by a vacuum or by a combination of inert gas and halogen, and such operating atmospheres are well known.

In one method of making the non-sag filament of the present invention, a very small amount of boron (isotope B is mixed with the tungsten powder prior to forming a compact of tungsten. As an example, the boron isotope can be included in an amount of about 0.5 percent by weight. The amount of added boron isotope does not appear to be critical. The formed tungsten compact is then pre-sintered by heating to a temperature such as 1,000C in a non-reactive atmosphere for about one-half hour. The pre-sintered compact is then self-resistance sintered in accordance with conventional techniques, in order to form a relatively massive sintered tungsten ingot. The sintering atmosphere is preferably hydrogen. The formed tungsten ingot is then swaged or otherwise mechanically worked until it is greatly elongated, with a relatively small diameter such as 0.083 inch (2.1 mm). The elongated material is then drawn to wire of desired size.

In accordance with the present invention, after the tungsten ingot has been mechanically reduced to a relatively small diameter, such as wire having a diameter of 0.1 inch (2.54 mm) it is passed through an irradiating chamber and irradiated with neutrons. The following nuclear reactions which occur can be described as follows:

After the foregoing irradiation, the tungsten is worked and drawn to wire of the desired size, and an example is 7 mils (0.178 mm) tungsten wire. The wire is formed into a coil or a coiled-coil as desired. This tungsten wire will exhibit a microstructure which comprises what is known in the art as a worked structure due to the swaging and the drawing operations, and the wire is ductile in that it can be wound into coils, with the formed helium atoms as well as lithium atoms distributed throughout the tungsten wire. When the tungsten filament is initially incandesced under predetermined controlled conditions, it will begin to recrystallize and the helium atoms, as well as residual lithium atoms, which are distributed throughout the tungsten will begin to coalesce to form small bubbles of helium and lithium. These bubbles will be discrete and will be generally aligned in the direction of the axis of the wire. As such, they will function similarly to the potassium bubbles of the standard doped tungsten and will retard recrystallization and control same due to the effect of dislocation and grain boundary pinning.

An enlarged sketch of the present wire is disclosed in FIG. 2 wherein the inert gas bubbles such as helium bubbles 22 are dispersed throughout the body of the tungsten filament 14, with a portion of the bubbles generally aligned along the axial dimensionof the filament and positioned at the boundries 24 of the overlapping elongated and interlocking grains 26 which comprise the filament. If alkali metal-formed bubbles 28 are also present, they will have the general appearance and orientation as the helium bubbles.

As a second method for forming the inert gas bubbles, after the filament has been drawn to a relatively fine size, such as a 0.1 inch (2.54 mm), for example, it is irradiated with alpha particles which forms helium atoms within the body of the tungsten. Thereafter, the filament is drawn to its desired size and then recrystallized to an interlocking structure.

In FIG. 3 is shown a transmission electron micrograph of alpha-particle irradiated pure tungsten which has later been annealed at a temperature of 1 ,900C for approximately 45 minutes, with the magnification in this electron micrograph being 60,000. Each of the plurality of helium bubbles which are shown have a diameter of approximately 200A. The helium atoms were introduced into the tungsten by irradiating tungsten ribbon with 30 MeV alpha particles. The helium bubbles began to nucleate or coalesce at an annealing temperature of approximately 1,600C and the bubbles continued to grow in size as the annealing temperature was increased. After an anneal at approximately 2,200C, the average size of the bubbles increased somewhat with the majority of the bubbles having a diameter substantially less than one micron.

To show the difference between tungsten which had been irradiated with the alpha particles to form the helium atoms therein, as compared to substantially pure tungsten which contained no helium, a narrow zone of a tungsten ribbon was irradiated with a 30 MeV alpha particles, with the resulting structure shown in FIG. 4, which is a photomicrograph taken at a magnification of 1,000 X. The irradiated zone occurs in this center of this photomicrograph and comprises an elongated, interlocking crystal structure comprising a plurality of elongated and interlocking grains defined by boundries therebetween. A very large number of very minute individual helium bubbles are included within the tungsten, and a portion of the bubbles are generally aligned along the grain boundries. The portion of the tungsten which was not irradiated with the alpha particles began to recrystallize at annealing temperatures in the order of 1,300 to 1,400C, but the recrystallization process was retarded in the irradiated band and initiation of recrystallization was not observed until an annealing temperature 2,200C was reached.

The concentration of the helium bubbles within the tungsten body does not appear to be a particularly critical and can vary over a wide range. As an'example, for helium bubbles having a diameter generally in the order of 2,600A, a bubble concentration per cubic centimeter of tungsten in the order of from about 4 X 10 to 5 X 10 provides very good results.

Inert gases other than helium can be formed within the tungsten body. As an example, if *Mg is included within the tungsten body and irradiated with neutrons, it will form within the tungsten a mixture of neon, he-

lium and the alkali metal sodium. The latter will form ture of lithium and sodium. The alkali metal-formed bubble will be present in very large numbers and will be of a very minute size, such as in the order of 100 to 500 A. As another example, Al irradiated with neutrons will form Na and helium atoms. Detailed expla: nations of such nuclear transmutations are described in Nuclear Physics (2nd Ed.), by Irving Kaplan, Addison- Wesley Publishing Co., Inc. (1964).

We claim:

l. The method of controlling the recrystallization of an elongated incandescent tungsten filamentary member to cause said member to retain a desired configuration under prolonged incandescent operating condi tions, said member having been formed into an elongated configuration by mechanically reducing in crosssection a relatively massive sintered tungsten ingot, said method comprising:

a. after said relatively massive sintered ingot has been b. after said elongated filamentary member has been fabricated into the form intended for ultimate use and also mounted in the environment in which'it is intended to operate, heating said filamentary member to a condition of incandescence to cause said inert gas atoms to coalesce into a very large number of very minute inert gas bubbles and to cause said filamentary member to recrystallize.

2. The method of controlling the recrystallization of an elongated tungsten filamentary member to cause said member to retain a desired configuration under prolonged incandescent operating conditions, which method comprises:

a. mixing with tungsten powder a very small amount of B forming the mixed powder into a compact, pre-sintering the compact in a non-reactive atmosphere, and self-resistance sintering the presintered compact in a hydrogen atmosphere to form a relatively massive sintered tungsten ingot;

b. mechanically reducing the sintered ingot to a relatively small diameter material;

c. irradiating the mechanically reduced material with neutrons to form helium atoms and lithium atoms from said B (1. further mechanically reducing the irradiated material to filamentary wire of the desired size, and forming the filamentary wire into a filamentary member of desired final form; and

etheating the formed filamentary member to a condition of incandescence to cause said helium atoms to coalesce into a very large number of very minute helium gas bubbles and to cause said filamentary member to recrystallize.

3. The method as specified in claim 2, wherein said B is mixed with said tungsten powder in amount of about 0.5 percent by weight thereof.

4. The method of controlling the recrystallization of an elongated incandescent tungsten filamentary member to cause said member to retain a desired configuration under prolonged incandescent operating conditions, said member having been formed into an elongated configuration by mechanically reducing in crosssection a relatively massive sintered tungsten ingot, said method comprising:

a. irradiating said elongated tungsten member with alpha particles to form helium atoms therein, and

b. after the irradiated member has been fabricated into a filamentary member having the form intended for ultimate use and also is mounted in the environment in which it is intended to operate, heating said filamentary member to a condition of incandescence to cause said helium atoms to coalesce into a very large number of very minute helium gas bubbles and to cause said filamentary member to recrystallize. 

2. The method of controlling the recrystallization of an elongated tungsten filamentary member to cause said member to retain a desired configuration under prolonged incandescent operating conditions, which method comprises: a. mixing with tungsten powder a very small amount of B10, forming the mixed powder into a compact, pre-sintering the compact in a non-reactive atmosphere, and self-resistance sintering the pre-sintered compact in a hydrogen atmosphere to form a relatively massive sintered tungsten ingot; b. mechanically reducing the sintered ingot to a relatively small diameter material; c. irradiating the mechanically reduced material with neutrons to form helium atoms and lithium atoms from said B10; d. further mechanically reducing the irradiated material to filamentary wire of the desired size, and forming the filamentary wire into a filamentary member of desired final form; and e. heating the formed filamentary member to a condition of incandescence to cause said helium atoms to coalesce into a very large number of very minute helium gas bubbles and to cause said filamentary member to recrystallize.
 3. The method as specified in claim 2, wherein said B10 is mixed with said tungsten powder in amount of about 0.5 percent by weight thereof.
 4. The method of controlling the recrystallization of an elongated incandescent tungsten filamentary member to cause said member to retain a desired configuration under prolonged incandescent operating conditions, said member having been formed into an elongated configuration by mechanically reducing in cross-section a relatively massive sintered tungsten ingot, said method comprising: a. irradiating said elongated tungsten member with alpha particles to form helium atoms therein, and b. after the irradiated member has been fabricated into a filamentary member having the form intended for ultimate use and also is mounted in the environment in which it is intended to operate, heating said filamentary member to a condition of incandescence to cause said helium atoms to coalesce into a very large number of very minute helium gas bubbles and to cause said filamentary member to recrystallize. 